In receiver circuits for microwave wireless communication systems, the low noise RF amplifier and mixer circuits may be formed on a single integrated circuit chip in order to simplify production and reduce costs. An example of a monolithic integrated active mixer/low noise amplifier (LLA) is described in IEEE MTT-SDigest (1995), pages 527 to 530, “A Novel 3V, 7 mA PHEMT GaAs active MMIC Mixer/LNA for Wireless Applications”, Henrik Morkner, Mike Frank, and Bryan Ingram. The circuit elements include pseudomorphic high electron mobility transistor (PHEMT) structures, formed using molecular beam epitaxy (MBE). The mixer/LLA is designed to operate over a frequency range of 900 MHz to 2,400 MHz, and the output of the amplifier is connected to the mixer through impedance matching circuitry, provided externally of the IC (i.e. off chip) to allow flexibility in selecting appropriate impedance matching circuitry for the desired operating frequency. External impedance matching circuitry is also provided for the intermediate frequency (IF) and local oscillator (LO) ports of the mixer and the RF input of the amplifier. Fabricating the mixer/LLA circuitry using a GaAs PHEMT provides enhanced gain and noise figure performance at low DC levels, but is inherently more expensive than silicon or regular GaAs fabrication techniques. Therefore, the provision of external impedance matching for each of the amplifier and mixer ports assist in minimizing the die area required for the receiver circuitry and minimizing cost.
Another example of an integrated MMIC receiver includes a pair of mixers coupled to the amplifier by a hybrid coupler to provide image rejection of the input RF frequency. The hybrid coupler is formed on the MMIC chip and splits the output signal from the amplifier into in-phase and quadrature RF signals which are fed to the image-reject mixers for mixing with a local oscillator signal. Examples of integrated receiver circuits having an image-reject mixer are disclosed in IEEE Microwave and Millimetre Wave Monolithic Circuits Symposium 1993, “A Highly Compact Wideband GaAs HEMT X-Ku Band Image-Reject Receiver MMIC, R. Katz et al, pages 131 to 134, and in “MIMIX Broadband, 17.0 to 27.0 GHz GaAs MMIC Receiver” at www.mimixbroadband.com.
As mentioned above, GaAs is preferred over silicon for integrated MMIC circuits, but the fabrication process is more expensive. Another drawback of GaAs fabrication is that it generally produces a lower yield of usable devices, which also leads to a higher cost of the end product.